It is desirable for electrically operated valves in operating facilities in chemical, petrochemical and power industries to be provided with fail-safe mechanisms to open or close upon loss of electrical power or instrument control signal at the actuator. In various types of piping systems in these facilities, it is even essential to have valves fail in a "safe" position for safety, to avoid system damage, or to avoid loss of chemicals. For example, in many systems, hazardous chemicals must be contained or controlled, or reactions must be prevented or stopped, in the event of system failure. There are many types of fail-safe valve systems known in the art, such as electro-pneumatic, electro-hydraulic and electromechanical systems.
One type of electromechanical system known in the art comprises an electric valve actuator and a spring return mechanism. This actuator uses a motor and gearbox to actuate a valve and to wind up a spring which reverses upon loss of electrical signal or power to the actuator. The actuator can operate to hold the spring in the open position continuously or to actuate the spring with every turn. Actuators which energize the spring during every actuation have a disadvantage of increasing the power requirements to run the actuator motor to twice that required to run a motor of an actuator not having a fail-safe mechanism. If the spring is instead continuously held in the energized position, it is not exercised until it is called upon to actuate the valve, thus, increasing the likelihood of sticking components and actuator malfunction.
Alternatively, electric valve actuators use a battery backup to reverse actuator direction when a loss of power is detected at the actuator. However, batteries will only actuate the valve a limited number of times depending on the storage potential of the battery and the power requirements of the actuator. Failure to maintain the battery charged and in good working order will negate the fail-safe feature of the actuator.
A third way of providing fail-safe mechanisms for electrically actuated valves is to use a spring return arrangement whereby fluid is pumped into a chamber by a hydraulic pump driven by an electric motor. The fluid moves a piston(s) and rotates the output shaft, while compressing and energizing the return spring(s). An electrically controlled solenoid valve maintains the pressure developed in the chamber. Upon loss of power or electrical signal, the solenoid valve is opened which allows the spring(s) to return the fluid from the pressurized chamber to the nonpressurized reservoir.
Examples of prior art electro-hydraulic valve actuators with spring returns are disclosed in U.S. Pat. No. 4,132,071, U.K. patent 2,005772 and Canadian patent 2,202,821. These electro-hydraulic actuators operate in similar manners whereby hydraulic fluid is pumped into a chamber which moves a piston against a spring. Release of the pressure allows the spring to return the piston and move the fluid from the pressurized center to the un-pressurized endcap or spring housing.
Prior art electro-hydraulic actuators such as those mentioned above have several disadvantages, including: 1) using scotch yoke and spur gear systems with a piston rod for changing linear motion in the actuator to rotary output motion, thus limiting rotary motion to a maximum of 90 degree; 2) using external conduits which are susceptible to freezing, breakage and leakage; 3) using external cylinders to house fail safe springs, thus requiring rod spring guide systems to insure linear compression of the spring without distortion; and 4 using only one solenoid valve for both fail-safe operation and position controlling.